1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus for use in laser printers, digital copiers, plain paper facsimile machines, and the like.
2. Description of the Related Art
More and more electrophotographic image forming apparatuses for use in laser printers, digital copiers, plain paper facsimile machines, and the like now provide color and high-speed printing, and tandem type image forming apparatuses having plural photoreceptors (typically, four photoreceptors) have become common. There are electrophotographic image forming apparatuses that have only one photoreceptor and are configured to rotate the photoreceptor the same number of times as the number of colors (e.g., in the case of four colors and one drum, the drum needs to be rotated four times). These color electrophotographic image forming apparatuses of this type are slow in copying speed. On the other hand, the tandem type image forming apparatuses have an increased number of light sources, resulting in an increase in the number of component parts, color shift due to the difference between wavelengths of the plural light sources, and higher costs. Furthermore, degradation of a semiconductor laser causes a writing unit to malfunction. The higher the number of the light sources, the higher the risk of malfunction. Especially, in the case where VCSELs or edge emitting LD array is used as the light source, the risk of malfunction is increased.
FIGS. 8A-8C are diagrams each illustrating a part of a related-art optical scanning device. FIG. 8A illustrates a two-stage rotating polygon mirror 1. FIG. 8B illustrates a method of making a beam from a single light source 2 incident on both stages 1a and 1b (upper and lower portions 1a and 1b) of the rotating polygon mirror 1. FIG. 8C illustrates a specific example of a beam splitter 4.
Referring to FIGS. 8A-8C, reference numeral 1 denotes the two-stage rotating polygon mirror (hereinafter referred to simply as a “polygon mirror”) that includes phase-shifted upper and lower reflective surfaces 1a and 1b (upper and lower portions 1a and 1b); 2 denotes the light source for writing; 3 denotes a collimator lens; 4 denotes the beam splitter; and L, L1, and L2 denote beams.
Reference numeral 4a denotes a semitransparent mirror; 4b denotes a total reflection mirror; 4-1 denotes a triangular prism; 4-2 denotes a parallelogram prism, 4-2a denotes an entrance window; and 4-1b and 4-2b denote exit windows.
In FIGS. 8A-8C, the vertical direction of the paper may be called a sub scanning direction, and the direction perpendicular to the paper may be called a main scanning direction.
An optical scanning device having a reduced number of light sources and capable of outputting images at high speed as shown in FIGS. 8A-8C is disclosed in Patent Document 1, for example. This optical scanning device is configured to split a beam of a common light source into plural beams and make the beams incident on different stages of a reflection mirror to scan different scanning surfaces.
In FIGS. 8A-8C, a beam L is emitted from the light source 2, is collimated by the collimator lens 3, and becomes incident on the beam splitter 4. The beam splitter 4 may employ various techniques based on different principles. Typically, a half mirror prism using the semitransparent mirror 4a as shown in FIG. 8C is used. The half mirror prism (beam splitter) 4 is formed by boding a side surface of the triangular prism 4-1 and a surface of the parallelogram prism 4-2 with the semitransparent mirror 4a. 
The beam L that has become incident on the entrance window 4-2a and passed through the semitransparent mirror 4a becomes a beam L1 having half the power of the beam L. The beam L1 goes straight to come out of the exit window 4-1b and becomes incident on the upper reflective surface (upper portion) 1a of the polygon mirror 1. The beam L that is reflected by the semitransparent mirror 4a becomes a beam L2 having half the power of the beam L and is reflected to the total reflecting mirror 4b disposed at the lower side. The beam L2 is made parallel to the original beam L by the total reflecting mirror 4b, comes out of the exit window 4-2b, and becomes incident on the lower reflective surface (lower portion) 1b of the polygon mirror 1.
The upper and lower portions 1a and 1b of the polygon mirror 1 are arranged with a phase difference, i.e., an angular difference θ. In this example, the angular difference θ between the upper and lower portions 1a and 1b of the four-faced polygon mirror 1 is 45 degrees.
Although not shown, a cylindrical lens having power in the sub scanning direction is disposed between the light source 2 and the polygon mirror 1. Further, although not shown, an imaging optical system that focuses the scanning light from the polygon mirror 1 onto the scanning surface, i.e., a photoreceptor is provided.
FIGS. 9A and 9B are diagrams illustrating problems with lights reflected by the upper and lower portions 1a and 1b, respectively, of the polygon mirror 1. FIG. 9A illustrates the case where the beam L1 incident on the upper portion 1a of the polygon mirror 1 scans a first photoreceptor (not shown). FIG. 9B illustrate the case where the beam L2 incident on the lower portion 1b of the polygon mirror 1 scans a second photoreceptor (not shown).
According to this configuration, while the upper beam L1 scans the surface (scanning surface) of the first photoreceptor, the lower beam L2 is preferably blocked by a light shielding member to prevent the beam L2 from reaching the scanning surface of the first photoreceptor. On the other hand, while the lower beam L2 scans the surface (scanning surface) of the second photoreceptor, the upper beam L1 is blocked to prevent the beam from reaching the scanning surface of the second photoreceptor. That is, the beam L1 and the beam L2 are alternately used. Accordingly, modulation driving of the light source 2 is performed at different timings for the upper portion 1a and the lower portion 1b. More specifically, during scanning of the first photoreceptor corresponding to the upper portion 1a, the modulation driving of the light source 2 is performed based on image information of a color (e.g., black) corresponding to the upper portion 1a. During scanning of the second photoreceptor corresponding to the lower portion 1b, the modulation driving of the light source 2 is performed based on image information of a color (e.g., magenta) corresponding to the lower portion 1b. 
In this system, because, for example, the semitransparent mirror 4a is used, the beam of the common light source 2 is split into two beams so that the power of each of the beams has about half the power of the beam before splitting, and thus the actual efficiency of the light source power is reduced. Therefore this system requires twice the power of a system using plural light sources or greater. Increasing the power leads to degradation of the light source 2, which may cause a writing unit to malfunction.
Although not described in detail, a combination of diffractive optical elements may be used as the beam splitter 4 for splitting the beam from the common light source 2 in place of the half mirror prism.
FIGS. 10 and 11 are timing charts of light emission wherein a single light source is used for scanning for two colors.
In the examples shown in FIGS. 10 and 11, light emission is performed for black and magenta.
In FIGS. 10 and 11, the solid lines indicate exposure for black and the dotted lines indicate exposure for magenta. Each waveform indicates exposure for one scanning line. The timing of starting writing is determined by detection of a scanning beam by a synchronous light receiving unit (not shown), which is disposed outside the effective scanning width. A photo diode is typically used as the synchronous light receiving unit.
In FIG. 10, the light intensity for the area of black is the same as the light intensity for the area of magenta. However, because optical elements relatively differ in transmittance and reflectance, if the light intensity of the light source for the area of black is the same as that for the area of magenta, the light intensity of the beam that reaches the photoreceptor for black differs from the light intensity of the beam that reaches the photoreceptor for magenta. To avoid such a problem, as shown in FIG. 11, different light intensities are used for scanning different photoreceptor surfaces, thereby equalizing the light intensities of the beams that reach the different photoreceptor surfaces.
FIG. 12 is a diagram illustrating a light emitting surface of a VCSEL (vertical-cavity surface-emitting laser) 21 as an example of a multibeam array.
In FIG. 12, reference numerals 20 and 21 indicate the VCSEL (see Non-Patent Document 1) and light emitters, respectively.
In the example of FIG. 12, there are provided 10 light emitting points in the horizontal direction by 4 light emitting points in the vertical direction, a total of 40 light emitting points. The light emitting points are disposed in slightly different positions from each other with respect to the end face of a substrate such that the 40 beams are spaced at the same interval upon drawing using multibeam wherein the horizontal direction is the main scanning direction.
Each light emitting point is formed in a square having sides parallel to the end face of the substrate, and a polarization plane is formed in the direction of these sides.
Such a light source is very expensive, and it is not preferable to use a large number of such light sources in view of the cost. As long as the optical system as described above is used, beam power loss is inevitable. To cover the power loss, it is necessary to increase the output of the light source or increase the number of light sources.
<Patent Document 1> Japanese Patent Laid-Open Publication No. 2005-92129 (corresponding to U.S. Patent Application Publication No. 2005/0099663A1)
<Non-Patent Document 1> Takashi Mori, Yasuhiro Yamayoshi, and Hitoshi Kawaguchi “Low-switching-energy and high-repetition-frequency all-optical flip-flop operations of a polarization bistable vertical-cavity surface-emitting laser” APPLIED PHYSICS LETTERS 88, 101102, 2006.